Organic electroluminescent devices used in organic electroluminescent displays are light-emitting devices including two electrodes and an emissive layer between the electrodes. In an organic electroluminescent device, electrons and holes are injected into an emissive layer through an electron injection electrode (or a cathode) and a hole injection electrode (or anode), and the electrons and holes recombine to form excitons which generate light while decaying from an exited state to a ground state.
Generally, organic electroluminescent devices have a two or three layer structure. Two-layer organic electroluminescent devices may include a hole transfer (injection) layer and an electron-transferable emissive layer. Three-layer organic electroluminescent devices may include a hole transfer (injection) layer, an emissive layer, and an electron transfer (injection) layer.
One of main tasks relating to organic electroluminescent devices is to improve efficiency when extracting light from an emissive layer. In an organic electroluminescent device, some of the light generated in an emissive layer and exiting from the emissive layer at an exit angle equal to or greater than a critical angle is totally reflected according to the refractive index of the emissive layer, and thus may not be extracted from the emissive layer. For example, if the refractive index of an emissive layer is 1.6, only about 20% of the light generated in the emissive layer may be extracted from the emissive layer.
Light scattering layers including light scattering particles have been introduced to improve light-extracting efficiency. For example, an optical waveguide layer disposed between a glass substrate and an emissive layer and including white fine particles dispersed in a transparent polymer matrix has been introduced (Japanese Patent Application Publication No. 2002-260844). In another example, an optical waveguide is disposed between a glass substrate and an emissive layer, wherein the optical waveguide includes transparent fine particles dispersed in a transparent polymer matrix and having a refractive index different from that of the transparent polymer matrix (Japanese Patent Application Publication No. 2002-260844).
Metal oxide particles may be used as light scattering particles. When metal oxide particles are dispersed into a matrix of a light scattering layer, the refractive index of the light scattering layer may be changed. In addition, light may be scattered at boundaries between the matrix of the light scattering layer and the metal oxide particles. However, it is difficult to disperse metal oxide particles into a matrix of a light scattering layer. That is, metal oxide particles are not easily dispersed into a matrix of a light scattering layer. Thus, it is difficult to include metal oxide particles in a matrix of a light scattering layer at a high concentration (for example at a high concentration in vol %). The adhesion between metal oxide particles and a matrix of a light scattering layer is poor. Thus, it may be difficult to obtain a light scattering layer having desired characteristics by dispersing metal oxide particles into a matrix of the light scattering layer.